Classical bits, qubits and qudits as well as square bits (or
squits, for short) are other unrestricted GPTs for which identical
arguments also result in a smaller set of axioms by means of our Gleason-type
theorem. The state and effect spaces of bits and qudits were described
in Section REF  while a squit or
gbit [1]} is a GPT with a square state space
and an octahedral effect space, as illustrated in Figure REF .
Pairs of squits are often considered in the study of non-local correlations
since they are capable of producing the super-quantum correlations
of a PR-box [2]}.
